Energy efficient highway addressable remote transducer C8PSK modem

Abstract

A method and a Highway Addressable Remote Transducer (HART) soft modem device (SMD) for transmitting a C8PSK signal and receiving and demodulating an Analog signal in either FSK or C8PSK comprising a HART message is provided. A transmitter of HART-SMD improves efficiency by reducing the computation complexity with a novel table lookup technique. A demodulator of the HART-SMD receives the Analog signal, samples and converts the Analog signal into digital data at a predetermined digital sample rate. The demodulator implements a two-step phase estimation technique followed by a single step timing estimate to achieve an optimal operating region for the receiver so that the receiver can be ready to receive state in a very short time as required in C8PSK specifications. A simple method of doing automatic modulation classification by negating the presence of an FSK modulation by the positive assertion of the presence of C8PSK modulation, or by the absence of C8PSK Signal and the presence of Energy detect to assert the presence of FSK signal is also stated.

We claim: 
     
         1 . A method for modulating and transmitting an Analog signal comprising a highway addressable remote transducer message, said method comprising of:
 a. Accepting as input said highway addressable remote transducer message and grouping them into 3 bits called symbols;   b. Further grouping sets of these symbols;   c. Implementing C8PSK modulation scheme in an efficient lookup table method to generate C8PSK signals which require only additions and subtractions and thus the requirements of computations are minimized significantly without need for any real time multiplication or other high power signal processing to generate said C8PSK signals;   
       where such a lookup table is generated ahead of time for a specific sample rate in a non-real time environment by making use of the properties of the C8PSK carrier frequency being in synchronization with the C8PSK baud rate and thus minimizing the size of this lookup table and the final C8PSK signals generated by a lookup table vector comprising of 3n bits representing the n symbols in time and m bits representing the number of C8PSK signals per symbol. 
     
     
         2 . The method for modulating and transmitting C8PSK signals recited in  claim 1 , wherein the value of n and m in a Lookup table is 3 which results in a look table size of 4096 bytes. 
     
     
         3 . The method for modulating and transmitting an C8PSK signals recited in  claim 1 , where the output sample rate is 12800 Hz or 25600 Hz. 
     
     
         4 . A method for efficiently receiving and demodulating a C8PSK signal as recited in  claim 1 , wherein the estimation of the Phase Error of the incoming C8PSK signal is carried out in the steps of;
 a. Sampling the incoming Analog C8PSK signal at a specified rate and generating I and Q signals twice per baud rate of 1/T seconds.   b. Estimating the incoming Phase Error to be within a specific Quadrant by inspecting sign values of certain measured I and Q values and further reducing the range to be within 45 degrees by magnitude comparison of certain measured values as;   c. Further applying the above measured first estimate of the correction for further collection of samples for the refinement of the phase Error Estimate to arrive at a final   d. Phase Error estimate and applying them cumulatively to aid in the subsequent step of Equalizer timing estimate.   
     
     
         5 . The method for receiving and demodulating a C8PSK signal as recited in  claim 4 , wherein the phase error estimates are calculated for 0, 45, 90, 135, 180, 225, 270 and 315 degrees and the final Phase Error Estimate is calculated using the approximation tan(θ)=θ for small angles and approximating the quotient itself as an estimate of arc tan minimizing calculations required. 
     
     
         6 . The method for receiving and demodulating a C8PSK signal as recited in  claim 4 , wherein the signal sampling rate is 12800 Hz and a T/2 equalizer is deployed, where T= 1/3200 Hz. 
     
     
         7 . The method for receiving and demodulating a C8PSK signal as recited in  claim 4 , wherein the timing estimation is sequentially done with the Phase Error estimation and estimation of the precise timing and selection of the equalizer is done in a single step using a decision estimator based on some measured properties of the I and Q signals after the Phase Error correction has been applied. 
     
     
         8 . The method for receiving and demodulating a C8PSK signal as recited in  claim 4 , wherein the equalizer selection is mapped to one of four equalizers. 
     
     
         9 . The method for receiving and demodulating a C8PSK signal as recited in  claim 4 , wherein the choice of timing is mapped one of two timing choices i.e. 0 or T/2 where T= 1/3200 Hz. 
     
     
         10 . The method for receiving and demodulating a C8PSK signal as recited in  claim 4 , wherein the automatic modulation classification is done by negating the presence of an FSK modulation by the positive assertion of the presence of C8PSK modulation or by the absence of C8PSK Signal and the presence of Energy detect to assert the presence of FSK signal. 
     
     
         11 . The method for receiving and demodulating a C8PSK signal as recited in  claim 4 , wherein the timing sequences of methods finishes in time with the correct choice of the demodulation and the demodulator being in proper state to receive the message in either C8PSK or the FSK modes.